1. Field of the Invention
This invention relates in general to the production of high purity materials suitable for use in diode-type applications such as solar cells and other semiconductor devices. The invention relates more particularly to the chemical vapor deposition (CVD) of silicon onto a heated substrate as well as apparatus therefor.
2. Description of the Prior Art
Methods for the production of high purity semiconductor materials are known. Most significant among those methods is the pyrolytic decomposition of chlorosilanes and silane to deposit silicon on resistively heated carrier rods in an inert, usually H.sub.2 atmosphere. Hydrogen and, where chlorosilanes are used, HCl are by-products. This method has been carried out in the so-called "Siemens" reactor, a quartz bell jar having one or more carrier rods of silicon, heated to red by the passage of electric current therethrough. The silane or chlorosilane decomposes at the reactor temperature and deposits silicon, usually polycrystalline silicon, onto the carrier rod or rods. The gas stream is heated to facilitate deposition. Sometimes silicon is deposited on the quartz walls. In a later improvement, cooling is provided at the walls to limit deposition thereon.
Many horizontal and vertical CVD apparatus use laminar flow with low conversion and a relatively short contact time of the deposition gas. Highly turbulent areas such as eddys are avoided because they increase non-uniform deposition and gas phase nucleation, the latter causing many problems including dusting on surfaces and reactant loss. Thus no study reports on deposition with a Reynold's Number for the gas stream above about 1600. The present commercial practice is restricted to laminar flow and low conversion during a long contact time. A gas is delivered at either a slow flow rate, or in the current commercial Siemens process, at effectively a zero flow rate for a sufficiently long residence time to effect decomposition and deposition.
The semiconductor grade silicon from chlorosilane decomposition is of very high purity, i.e., with the substantial absence of electrically active contaminants such as boron and phosphorus. Since boron and phosphorus, among other metal contaminants, are normally found in silicon source materials such as quartz, their elimination at every opportunity is important. Purification of quartz (predominantly SiO.sub.2) to metallurgical grade silicon results in about 95-99% purity. The metallurgical grade silicon is used as a source material to make halosilanes or silane of high purity. Chemical vapor deposition and subsequent processing often results in silicon purity of 99.9999%. The subsequent processing is usually either Czochralski seed-pulling from a melt of silicon or float-zone processing techniques, both well known and widely used in the art. Other new techniques including ribbon production and edge-defined growth technique (EFG) are also used to prepare single crystal silicon for solar cell or other semiconductor device applications. Single crystal ingots are sawed into wafer shapes and polished.
One of the problems of some prior art systems is "dusting" caused by nucleation of silicon in the gas stream (creating silicon dust) rather than at the deposition surface.
Since the semiconductor industry is growing at such a rapid rate and with fierce competition, there exists a need to lower the cost of production. One area where cost reduction is sought is the chemical vapor deposition of polycrystalline silicon and other semiconductor materials.
Despite the fact that productions of silicon by known processes is very expensive, a great demand exists for polycrystalline silicon product.